Thermoplastic resin films, especially those prepared from polyesters, have been widely used as condensers, packaging and labelling materials, photographic films, magnetic recording media and the like. For such various applications, it is desired that they have uniform physical and chemical properties in their transverse direction. When such film properties as heat shrinkage, mechanical strength and density are not uniform in the transverse direction, various difficulties are likely to occur in the further processing of the film, such as coating, deposition, slitting or printing, thereby deteriorating the quality of the film product. Therefore, numerous attempts have been made to produce a film having uniform physical and chemical properties in the transverse direction.
Thermoplastic resin films are generally prepared by a process of melting and extruding a thermoplastic polymer chip into an amorphous sheet, extending the sheet in longitudinal and transverse directions and then heat treating the extended film. In the process, the transverse extension and heat treating is carried out in a tenter and plays a critical role in maintaining the uniformity of the film properties in the transverse direction. In producing a film in accordance with the conventional methods, "bowing" of film tends to occur during the transverse extension and heat treating, which may cause a non-uniformity in the transverse directional properties thereof.
Said bowing refers to a phenomenon caused by a difference in the deformation between both edges and the center of the film being processed. That is, after a longitudinal extension, the film is extended along the transverse direction with its both edges held by, e.g., clips, and then heat treated. During the transverse extension and heat treatment, therefore, the edge areas of the film remain essentially intact due to the stability force of the clips, while the center of the film may be greatly deformed. Such difference in the degree of deformation depending on the transverse location of the film causes a bowing effect.
FIG. 1 shows such bowing effect of a film occurring during a transverse extension and heat treating process in a tenter. When the film, after being drawn in a transversely straight line, is extended in the transverse direction and heat treated, it is deformed in the shape of a bow as illustrated. In FIG. 1, b shows the extent of the deformation, W is the width of the film, and the arrow means the proceeding direction of the film. Such bowing effect results in the non-uniformity in the film properties along the transverse direction.
Consequently, extensive attempts have been made to prevent the bowing. For example, Japanese Patent Laid-open Publication No. 64-29214 describes a heat treatment by the use of heating rolls. In accordance with the process, however, both edges of the film are heat treated by heating rolls which are not firmly fixed or stabilized, thereby causing a shrinkage in the transverse direction.
Japanese Patent Laid-open Publication No. 67-9273 discloses a process of heat treating a film under a temperature gradient in the transverse direction; and Japanese Patent Laid-open Publication Nos. 87-183327 and 183328 offer the methods of heat treating a film with a heating gas, the flow rate (or amount) of which is different in the transverse direction. However, these methods have the disadvantages that require the use of complicated equipment and a substantial period for adjusting the conditions.
Japanese Patent Laid-open Publication No. 75-73978 presents a method including an additional heat treatment of a film with nip rolls between the transverse extending process and the heat treating process; and Japanese Patent Laid-open Publication No. 88-24459 discloses a method of heat treating while the center of the film is proceeded forcibly by nip rolls. These methods have the disadvantages in that the film may be damaged by the contact with the hot rolls.
Moreover, Japanese Patent Laid-open Publication Nos. 91-130126, 91-130127, 92-142916 and 92-142917 describe methods of inserting a cooling process after the transverse extension. However, these methods have limited effects, if any, while requiring the use of an additional apparatus.